It is now well known to utilize a piece of low coercive force, high permeability magnetic material as the responder material in a harmonic generating EAS marker. Such markers were perhaps first disclosed in the French Patent No. 763,681, issued in 1934 to Pierre Arthur Picard. More recently, it has become relatively well known to use particularly configured pieces, such as elongated strips of high permeability material, as set forth in U.S. Pat. Nos. 3,665,449, 3,790,945 and 3,747,086. It is also known from the latter disclosures to provide dual status markers by including at least one piece of a permanently magnetizable material adjacent to the piece of high permeability material. When such additional pieces are magnetized, an external magnetic field is present which biases the high permeability responder material, thereby altering the response of the marker in an alternating interrogation field.
While still recognizing that an elongated, or "open-strip" configuration is desired in order to obtain a very high order harmonic response, U.S. Pat. No. 4,075,618 (Montean) discloses that a marker capable of generating very high order harmonics, thereby being operative in a system such as described in the '449 patent, may be made by adding flux collectors to a short strip of high permeability material which is insufficiently long to meet the definition of an "open-strip". That patent also suggests that a dual status flux collector type marker may be made by adding at least one piece of remanently magnetizable material adjacent to the material in the center section of the marker (i.e., between the flux collectors), which when magnetized biases the center section and alters the harmonic content of the signal produced by the center section.
Additional, non-elongated, dual status, flux collector type markers are disclosed in patent applications of Samuel Montean, U.S. Ser. No. 909,340 filed Sept. 19, 1986, now U.S. Pat. No. 4,710,754 and U.S. Ser. No. 909,467 filed Sept. 19, 1986 now abandoned, which are assigned to the same assignee as that of the present invention. All such disclosures propose only that keeper elements be provided adjacent to a center switching section of low coercive force responder material.
Typical EAS systems designed for use with the magnetic marker described above, are the Model WH-1000 and 1200 systems, marketed by Minnesota Mining and Manufacturing Company. Such systems typically produce within interrogation zones magnetic fields alternating at 10 kHz, and having minimum intensities at the center of the zone of approximately 1.2 oersteds (Oe), when fields generated in coils on opposite sides of the zone are in an opposing configuration, and of approximately 2.4 Oe when in an aiding configuration. The receiver portions of such systems process signals from receiver coils positioned within panels adjacent to the interrogation zone, and activate an alarm circuit in the event signals corresponding to very high order harmonics of the applied field are detected.
To compare the performance of various markers, it is convenient to use a test apparatus which generates fields alternating at a predetermined frequency and has controllable strength comparable to those encountered in such EAS systems. The test apparatus should detect signals in accordance with the harmonic characteristics relied upon in such systems and provide sensitivity values, based on a standard marker to ensure valid comparative results.
Such a test apparatus is preferably constructed to allow a marker to be inserted parallel with the field of the test apparatus and the gain adjusted to indicate a standardized sensitivity value.